This invention relates to surface functionalized porous silicon substrates. More particularly, this invention is directed to porous silicon surfaces having covalently bound monolayers formed using optionally substituted alkenes and alkynes under exceptionally mild conditions using a solvent-soluble Lewis acid.
Silicon surface chemistry is of fundamental technical significance because of the ubiquitous role of silicon in modern technology, and yet it is only just beginning to be investigated. Most microprocessor chips in electronic products are based upon crystalline silicon wafers. Control of silicon surface chemistry is crucial to allow access to technologically interesting thin films for fabrication of new electronic devices. In 1990, Canham and co-workers showed that silicon wafers could be etched to produce a microns-thick porous layer (termed porous silicon) that exhibits photoluminescence upon exposure to UV light [Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046]. Potential applications for porous silicon include uses as chemical sensors, biosensors, optoelectronic devices such as electroluminescent displays, photodetectors, and as a matrix for photopumped tunable lasers. As a result, modification and characterization of photoluminescent porous silicon surfaces has become an area of intense interest.
Hydrosilylation of olefins and alkynes has been known to proceed under a wide variety of reaction conditions. Late transition metal catalysts were commonly used in these reactions, however, such catalysts have the potential for activating the weaker Sixe2x80x94Si bonds on the surface (bond strengths: Sixe2x80x94Si=340 kJ/mol, Sixe2x80x94H=393 kJ/mol). Lewis acid catalyzed/mediated hydrosilylation reactions have also been reported. Aluminum chloride, for example, is known to be an effective catalyst for hydrosilylation of both alkynes and alkenes, but that Lewis acid is not soluble in non-polar solvents and, therefore, is not suitable for solid phase chemistry.
This invention provides a mild and general method for covalent modification of the surface of porous silicon through hydrosilylation of readily available alkynes and alkenes mediated by a Lewis acid such as EtAlCl2 that is soluble in non-polar solvents.
Hydrosilylation of alkynes and alkenes catalyzed by EtAlCl2 and other solvent-soluble Lewis acids by surface situated silicon hydride groups on a porous silicon surface smoothly yields vinyl and alkyl groups, respectively, covalently bound to the surface. The present method is tolerant of a variety of functional groups. Thus, for example, nitrile, hydroxy and methyl ester substituted olefins have been used to form covalently bound monolayers on porous silicon surfaces without additional protecting groups. The xe2x80x9csolvent solublexe2x80x9d type of Lewis acid used in accordance with this invention plays a dual rolexe2x80x94it mediates the hydrosilylation event, and it acts as a reversible protecting group for Lewis basic sites in the unsaturated reactant that can be removed after the reaction by washing with donating solvents.
This invention also provides porous silicon substrates having a surface comprising a covalently bound monolayer. Such porous silicon substrates are remarkably stable under a wide variety of conditions normally resulting in degradation of the delicate porous surface structure.